1. Field of the Invention
The present invention relates to a power supply device for electric discharge machining, and in particular to a power supply device in which charging and discharging of a capacitor for storing electrical energy for the electric discharge machining is controlled by switching elements.
2. Description of Related Art
As means of improving a machining speed using a capacitor-type machining power supply device which charges a capacitor and applies the voltage of the charged capacitor between a workpiece and an electrode to cause electric discharge to perform machining, there is known a capacitor-type power supply device for electric discharge machining comprising switching elements for controlling the charging and discharging of the capacitor. FIG. 3 is an example of the capacitor-type power supply device for electric discharge machining in which the control of the charging and discharging is performed by switching elements.
This power supply device for electric discharge machining is designed to charge a capacitor C from direct-current power source E via a first switching element (transistor) SW1 and a current limiting resistor R, and then apply the voltage of the charged capacitor C between an electrode P and a workpiece W via a second switching element (transistor) SW2 to cause electric discharge between the electrode C and the workpiece W to machine the workpiece W.
FIGS. 4a to 4d are operation timing charts in this power supply device for electric discharge machining. First, when the first switching element SW1 is turned on (FIG. 4a), a current flows from the direct-current power source E through the switching element SW1 and the resistor R to the capacitor C to charge the capacitor C, so that the voltage Vc of the capacitor increases as shown in FIG. 4b. When the first switching element SW1 is turned off and then the second switching element SW2 is turned on (FIG. 4c), the voltage Vc of the charged capacitor C is applied between the electrode P and the workpiece W, so that a discharge current Ig flows in between as shown in FIG. 4d to machine the workpiece.
This power supply device for electric discharge machining includes a resistor R to limit oscillation and a peak value of a current when the capacitor C is charged. The resistor R consumes energy. In principle, the resistor consumes as much energy as the capacitor C stores. Hence, the energy efficiency is low.
As a means of improvement, there is known a technique of liming a current using an inductor in place of a resistor (see JP60-180718A and JP 01-210219A).
FIG. 5 shows an example of a power supply device for electric discharge machining arranged to limit a charging current to the capacitor using an inductor. In this power supply device, an inductor L is connected in place of the resistor R in the power supply device shown in FIG. 3. Further, a diode D is connected in a reverse direction to be parallel with a series of direct-current power source E and a first switching element SW1. In the other respects, this power supply device is the same as the power supply device shown in FIG. 3.
FIGS. 6a to 6c are operation timing charts in this power supply device for electric discharge machining until the capacitor C is charged. When the first switching element SW1 is turned on (FIG. 6a), a current from the direct-current power source E flows through a closed loop comprising the first switching element SW1, the inductor L, the capacitor C and the direct-current power source E to charge the capacitor C, so that the voltage Vc of the capacitor increases as shown in FIG. 6c. When the first switching element SW1 is turned off, a current Id produced by energy stored in the inductor L flows through the diode D as shown in FIG. 6b to further charge the capacitor C. Specifically, a current Id produced by energy stored in the inductor L flows from the capacitor C, through the diode D, and then through the inductor L to charge the capacitor C, so that the voltage Vc of the capacitor further increases. Then, the second switching element SW2 is turned on to apply the voltage Vc of the capacitor between the electrode P and a workpiece W to cause electric discharge in between to perform machining.
The power supply device for electric discharge machining which limits a charging current using an inductor as shown in FIG. 5 hardly produces energy loss, and therefore it can be called an efficient power supply device for electric discharge machining. However, even when the first switching element for charging is tuned off, a current produced by energy stored in the inductor flows and charges the capacitor. Hence, it is difficult to control a charge voltage of the capacitor which is applied between the electrode and a workpiece. In order to control the charge voltage of the capacitor, it is necessary to accurately set an ON-time of the switching element by estimating charges supplied from the inductor, so that a control circuit of the switching element is complicated.